28 STEEL ROOF TRUSSES AND MILL BUILDINGS. CHAP. I. 



of planks, I to 3 inches thick. The planks are sometimes laid in two thicknesses with a layer of 

 lime mortar between the layers as a protection against fire. In fireproof buildings the sheathing 

 is commonly made of reinforced concrete. Concrete slabs are sometimes used for a roof covering, 

 being in that case supported directly by the purlins, and sometimes as a sheathing for a slate or 

 tile roof. 



The roofs of smelters, foundries, steel mills, mine structures and similar structures are com- 

 monly covered with corrugated steel. Where the buildings are to be heated or where a more 

 substantial roof covering is desired slate, tile, tin or a good grade of composition roofing is used, 

 or the roof is made of reinforced concrete. For very cheap and for temporary roofs a cheap com- 

 position roofing is commonly used. The following coverings will be described in the order given; 

 corrugated steel, slate, tile, tin, and tar and gravel. A slate roof on reinforced concrete sheath- 

 ing is shown in Fig. 45 and in Fig. 46. 



CORRUGATED STEEL ROOFING. Corrugated steel roofing is laid on plank sheathing or 

 is supported directly on the purlins. Corrugated steel roofing should be kept well painted with a 

 good paint. Where the roofing is exposed to the action of corrosive gases as in the roof of a smelter 

 reducing sulphur ores, ordinary red lead or iron oxide paint is practically worthless as a protective 

 coating; better results being obtained with graphite and asphalt paints. Tar paint, made by 

 mixing tar, Portland cement and kerosene in the proportions of 16 parts of tar, 4 parts of Portland 

 cement, and 3 parts of kerosene, by volume, is an excellent protection against corrosive gases in 

 smelters and similar structures. Galvanized corrugated steel is quite extensively used. To pre- 

 vent the condensation of vapor on the inside of the metal roof, corrugated steel roofing should 

 be laid on sheathing or should have anti-condensation lining. 



Corrugated steel sheets covered with an asbestos preparation can now be obtained on the 

 market. 



Anti-Condensation Lining. Anti-condensation lining, shown in Fig. 23, consists of asbestos 

 felt supported on wire netting that is stretched tight and supported by the purlins. Anti-con- 

 densation lining is put on according to two systems. 



Berlin System, (5) Fig. 23. (i) Lay galvanized wire netting, No. 19, 2-in. mesh, trans- 

 versely to the purlins with edges about i| in. apart so that when laced together with No. 20 brass 

 wire the netting will be stretched smooth and tight. When the purlins are spaced more than 4 ft. 

 apart stretch No. 9 galvanized wire across the purlins about 2 ft. centers to hold up the netting. 



(2) On the top of the wire netting place a layer of asbestos paper weighing 14 Ib. per square 

 of 100 sq. ft., and on this place a layer of asbestos paper weighing 6 Ib. per square. All holes in 

 the paper must be patched when laid. 



(3) On top of the asbestos paper lay two thicknesses of Neponset building paper. 



Note. The asbestos and building paper should lap 3 in. and break joints 12 in. The corru- 

 gated steel is fastened with the usual connections. Use tin washers on corrugated steel bolts 

 where there is danger of breaking or tearing the lining. 



Wire netting, No. 19 gage, 2-in. mesh comes in bundles 6 ft. wide and 150 ft. long, containing 

 900 sq. ft. Asbestos comes in rolls 36 in. wide and is sold by the pound. No. 20 brass wire is 

 bought by the pound, 272 lineal ft. weigh one pound. Neponset building paper conies in rolls 

 36 in. wide and 250 ft. or 500 ft. long. Do not cut a roll. Add 10 per cent for laps of asbestos 

 and building paper. 



Minneapolis System, (6) Fig. 23. (i) Lay wire netting, No. 19, 2-in. mesh, transversely to 

 the purlins, with edges I J in. apart, so that when laced together with No. 20 brass wire the netting 

 will be stretched smooth and tight. 



(2) On the top of the netting lay asbestos paper weighing 30 Ib. to the square of loo sq. ft., 

 allowing 3 in. for laps. For important work lay one or two thicknesses of building paper on top 

 of the asbestos. 



(3) Lay the corrugated steel and fasten to purlins in the usual manner. 



Note. If wood purlins are used the wire netting may be fastened to the nailing strips with 

 | in. staples. Where the purlins are more than 2 ft. 6 in. centers place a line of ^ in. bolts between 

 purlins, about 2 ft. centers, with washers I in. X 4 in. X | in. to prevent netting from sagging. 



' * SLATE ROOFING. Roofing slates are usually made from f to \ inches thick; ^ inch 

 being a very common thickness. Slates vary in size from 6 in. X 12 in. to 24 in. X 44 in.; the 

 sizes varying from 6 in. X 12 in. to 12 in. X 18 in. being the most common. 



